Superconductors are commonly known as electrically conducting materials that, below some critical temperature T.sub.c, which varies among materials, provide zero electrical resistance for currents imposed upon them. Known example materials are lead, niobium, and aluminum, which have approximate critical temperatures of 7.2, 9.2, and 1.2 degrees Kelvin, respectively. Various metal alloys are also known to exhibit superconductivity, and recently, metal oxides with critical temperatures near 90 degrees Kelvin have also been discovered.
Practical applications of superconductors have used these materials both in wire form and in thin film form. In film form, the thin films are typically deposited on dielectric substrates when used in electronic applications, and typically have thicknesses in the range of 1 nm to 1000 nm.
Superconductors with lower critical temperatures are commonly cooled by liquid helium, while superconductors with higher critical temperatures may be cooled with liquid nitrogen. Alternatively, superconductors can be cooled with various mechanical refrigerators.
The superconducting state in a material can be destroyed by raising the temperature of the material above its critical temperature, and this state can also be destroyed by passing too much electrical current through the material. The smallest electrical current which produces the onset of electrical resistance in a superconductor, is called its critical current. The critical current of a superconductor depends on the material from which it is made, on the temperature of that material, on the cross sectional shape of the material, and on its dimensions.
Conductors not in the superconducting state are said to be in the normal state. Electrical currents in wire made of normal metal are uniformly distributed across the cross section of the wire, if the current is at sufficiently low frequency. In contrast, current in a superconducting wire is almost exclusively in a very thin layer at the surface of the wire, for wire of ordinary diameter (0.1 mm or greater).
The magnetic field produced by this current is also constrained to this same thin surface layer, so far as its distribution within the superconducting material is concerned. The thickness of this layer is called the magnetic penetration depth. This depth is typically about 100 nm, but the depth depends upon temperature and varies with different superconducting materials. According to the two-fluid model of superconductors, the dependence of the penetration depth .lambda. on temperature is given by ##EQU1## where .lambda..sub.0 is the penetration depth at temperature T equal to zero, and T.sub.c is the critical temperature.
Sufficiently large electrical currents in a superconductor, but less than the critical current defined above, may introduce magnetic vortices into the superconductor. These vortices are permanently circulating "whirlpools"[of electrical current, with diameters of about twice the magnetic penetration depth. The presence of vortices considerably complicates the electrical response of a superconductor. For example, the movement of vortices within the superconductor produces an effective nonzero electrical resistance on a transient basis, and also a transient change in inductance of the superconductor. Consequently, it is normally desirable to avoid the introduction of vortices into the superconductor in this invention.
The electrical current in a superconductor in the form of a long uniform ribbon is concentrated near the opposite edges of the ribbon, with relatively small current near the center of the ribbon. If the plane of this ribbon is located near to (in comparison with the width of the ribbon) and parallel with a superconducting ground plane, then the current is distributed essentially uniformly and exclusively across the surface of the ribbon that is closest to the ground plane.
If the thickness of the ribbon is one magnetic penetration depth or less, then, in the presence of the ground plane, the current will be distributed essentially uniformly across the cross section of the ribbon of superconductor. This uniform current distribution provides the maximum current carrying capacity for a superconducting thin film without the introduction of the magnetic vortices.
The temperature dependence of both the resistance and the inductance of superconductors have heretofore been used as the bases for thermometers. The resistance based thermometer is operated at temperatures just above its critical temperature, where the superconductor has a strongly temperature dependence resistance, and, in fact, this device is not superconducting at the operating temperature.
The inductance based thermometer, on the other hand, is operated just below its critical temperature, where it has zero resistance, and a strongly temperature dependent inductance. One advantage of the inductive device is that it has a lower noise level because it does not have Johnson noise since its resistance is zero.
It has also heretofore been foundthat higher sensitivity to temperature is obtained with the inductive device if the superconductor is made very thin, so that the kinetic inductance dominates the magnetic inductance (see W. A. Little, "Device Application of Super-inductors", Proceedings of the Symposium on the Physics of Superconducting Devices, 1967, pages S1-S17, and R. Meservey and P. M. Tedrow "Measurement of Kinetic Inductance of Superconducting Linear Structures", Journal of Applied Physics, volume 40, pages 2028-2034, (1969)). In this prior work, a single layer of thin film superconductor was used as the temperature sensor.
In the work of J. W. Baker, J. D. Lejeune, and D. G. Naugle, entitled "Effects of a nonuniform current distribution on the kinetic inductance of a thin superconducting film", appearing in the Journal of Applied Physics, Vol. 45, pages 5043-5049 (1974), the temperature dependence of the kinetic inductance of a thin film superconductor over a superconducting ground plane was studied. The ground plane, however, was not a thin film (the ground plane had a thickness of 1 mm), and the device could therefore not have the same practical applications as could a device with a thin film ground plane, as brought out hereinafter, particularly for devices requiring thermal isolation of one part of a superconductor circuit from other parts of the circuit. Economic differences also favor use of a thin film ground plane.
An inductive superconducting thermometer has also heretofore been developed using a wire wound construction for the device (see M. V. Moody, H. A. Chan, H. J. Paik, and C. Stephens, "A Superconducting Penetration Depth Thermometer", Proceedings of the 17th International Conference on Low Temperature Physics, edited by U. Eckern, A. Schmid, W. Weber, and H. Wuhl (North-Holland, Amsterdam), 1984, pages 407-408). This device was fabricated by winding superconducting wire around a superconducting rod. In this device, magnetic inductance was more important than kinetic inductance (which was not mentioned in the discussion).
Several different devices for detecting electromagnetic radiation are well known, and among such well known devices are bolometric detectors. A bolometric detector is basically a thermometer (heretofore usually made of a semi-conductor material rather than a superconductor material) in good thermal contact with a material that absorbs electromagnetic energy, such as microwaves. In the simplest model of a bolometer, the absorber and thermometer are normally considered to be at the same temperature, and they are jointly connected to a thermal reservoir by an appropriate thermal conductance G. The thermal conductance G plays an important role in determining the noise level and sensitivity of the overall device, and the internal noise and sensitivity of the thermometer play an important role in the performance of the detector.
An approximate relation between the radiation power absorbed by the bolometer P.sub.a, and its change in temperature, is EQU P.sub.a =G(T-T.sub.0), (2)
where G is the thermal conductance, T is the temperature of the absorber-thermometer combination with the radiation applied, and T.sub.0 is their temperature without applied radiation. Thus, the measurement of power is reduced to a measurement of a change in temperature and a thermal conductance.
Bolometers have many applications, for example, the measurement of electrical and electromagnetic power across a very broad spectrum, ranging from audio frequencies through radio frequencies, microwaves, and millimeter waves, and encompassing the infrared and visible spectrum. Also, suitably designed bolometers based on the disclosed thermometer can measure the energy of optical and infrared pulses from lasers, and the energies of individual x-ray and .gamma.-ray photons, as well as elementary charged particles, such as protons.
A substantial and generally unique advantage of bolometers over other radiation detectors is that they can be designed to be calibrated power detectors. This is achieved by applying a known amount of dc power to the device and comparing its response with that for the absorbed radiation power.
In practice, the main change for detecting or measuring one kind of radiation as opposed to another, is a change in the absorber. Metal films of gold-chromium alloys, with thicknesses less than 100 nm, are a representative material for absorbing infrared radiation. The shape and thickness of the absorbing film must be adjusted to give the appropriate impedance, reflection coefficient, or absorption of the applied radiation, and the area of the absorber must be adjusted with due consideration to power handling capability.
Modern day practice for circuit design frequently emphasizes the development of electronic circuitry in thin film form on a single substrate, called integrated circuits. The electrical components in circuits now fabricated in this way are not readily adjustable, and ordinarily the user must be content with the resistive, capacitive, and inductive components as they are fabricated. If different values are required, then the circuit heretofore had to be fabricated anew.